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ADS1293: Calculate the "LSB size"

Christian Klerx
Prodigy 10 points
Part Number: ADS1293

Hello all,

I would like to calculate the "LSB size" of the ADS1293 at a certain configuration.

The data sheet says the following:

8.3.5 Instrumentation Amplifier (INA)
The instrumentation amplifier provides a high input impedance to interface with signal sources that may have relatively high output impedance, such as ECG electrodes. The maximum differential input voltage range of the Sigma-Delta Modulator (SDM) behind the INA is ±1.4 V, and the gain of the INA is 3.5x. Therefore, the maximum differential input voltage of the INA is ±400 mV.

and

8.4.3 Ouput Code (ADCOUT)
The ADCOUT of the ADS1293 is due to a differential voltage applied between the positive and negative input terminals of the instrumentation amplifier and can be calculated with Equation 13:
(13)
The reference voltage VREF, equals to 2.4 V if the on-chip voltage reference is used. ADCMAX represents the maximum output code of the ADC, which corresponds to a theoretical 2.4-V signal at the input of the SDM. The value of ADCMAX changes with the configuration of the digital filters, and the corresponding value can be found in Table 8, Table 9, Table 10, and Table 11. Note that ADCOUT equals ADCMAX/2 for a 0V differential input.

I am a bit confused by the different statements about the input voltages of the SDM in 8.3.5 (+/-1.4V=2.8V) and 8.4.3 (2.4V)
Is there an explanation for this?

Assuming a configuration with ADCMAX = 0xF30000 (from 8.4.3) and +/-400mV (from 8.3.5), this is how I would calculate the LSB size:

LSB = 800 mV / 0xF30000 = 50.23... nV/Digit

Is that correct?

Thanks for the Help

Christian

  • 0 in reply to ChienChun Yang
    Prodigy 10 points

    Hi,

    Thanks for the answer. Unfortunately the post in the link does not answer my question.

    My problem is that I don't know how the FSR of the ADS1293 is with the given configuration with ADCMAX= 0xF30000. Unfortunately the datasheet is misleading for me.

    It talks once about a signal of +/-1.4V (chapter 8.3.5) and once about "theoretical" 2.4V (chapter 8.4.3) at the input of the SDM. Calculated with the amplification of 3.5 there would be either +/-400mV, +/-343mV or even +/- 686mV at the input of the ADS1293.

    Where is my thinking error?
    What is correct?

    Is this the correct formula?
    LSB = 800 mV / (0xF30000-1) = 50.23.. nV/Digit

    Thanks
    Christian

  • 0 in reply to Christian Klerx
    TI__Mastermind 19815 points

    Hi,

    The app engineer is on business travel trip this week. Please expect delay.

    Apology for the inconvenience.

    Thanks.

  • 0 in reply to ChienChun Yang
    TI__Mastermind 19815 points

    Hi,

    ADS1293 sets differential voltage 0V to the half of the max ADC code, i.e. 1/2 of ADCMAX.

    Please follow datasheet 8.4.3 Ouput Code (ADCOUT) equation (13).

    VREF, equals to 2.4 V if the on-chip voltage reference is used.

    The value of ADCMAX changes with the configuration of the digital filters, and the corresponding value can be found in Table 8, Table 9, Table 10, and Table 11.

    Note that ADCOUT equals ADCMAX/2 for a 0V differential input.

    i.e. ADS1293 sets differential voltage 0V to the half of the max ADC code, i.e. 1/2 of ADCMAX.

    e.g. if one configure the ADS1293 to have ADCMAX = 0x800000 and use VREF = 2.4V.

    Then, if you want to know how much differential voltage(VINP-VINM) that LSB equals to, you could set the ADCOUT to (1/2)*ADCMAX) +1 in the equation 13 and replace the VREF and ADCMAX by the values mentioned above i.e. -

    ((1/2) *8388608 + 1) = 4194305 

    ((4194305/8388608) - 1/2) * (2*2.4)/ 3.5 = 1.63487026e-7 V.

    But, again, 0V is not ADC code 0s. 0V is 1/2 of the ADCMAX selected/set by user.

    Thanks